1. Field of the Invention
The present invention relates to a method for preparing metal nanoparticles, and specifically, to a method for more efficiently preparing metal nanoparticles with superior properties by reducing a metal precursor compound in an aqueous solution using polyethylenimine and additional reducing agent.
2. Description of the Prior Art
Metal nanoparticles are diversely utilized in electronics, optical sciences, catalysts, and biological fields, etc due to their physicochemical properties. Specifically, conductive metal nanoparticles can be used to prepare conducting films, and thus they are of high interest in the fields of smart windows, rewritable electronic papers, electronic panel displays, and flexible displays, etc.
These metal nanoparticles can be prepared by various methods including reduction-precipitation method in an aqueous solution, an electrochemical method, an aerosol method, a reverse microemulsion method, a chemical liquid phase deposition method, a photochemical reduction method, and a chemical reduction method in a solution, etc. However, such preparation methods are either very complicated or exhibit very low yield, and thus there has been a need for the development of an improved and novel method.
The existing method for preparing metal nanoparticles includes a seed mediated synthesis method developed by Murphy et al., one of the representative methods for preparing gold nanoparticles. In this method, NaBH4, a strong reducing agent, is used to form a gold cluster with a diameter of about 3 to 4 nm, and then allows the gold cluster to grow into nanoparticles by adding a seed to a hydrogen tetrachloroaurate acid solution containing ascorbic acid, which is a weak reducing agent. This method has disadvantages in that it requires preparation of many types of chemical materials, the process is very complicated, and the number of moles of a gold particle to be prepared per unit volume is limited to 2.5 mM, thus resulting in a very low yield.
To complement such disadvantages, a one-pot synthesis method (Hoppe et al.) using polymer (a representative example includes polyvinylpyrrolidone (PVP)) may be used because the preparation process is relatively simple. However, the problem of low yield still remains unsolved due to the limitation on the concentration of a gold precursor compound to 2 mM.
Also, in the case of silver nanoparticles, when ascorbic acid is used as an organic reducing agent in a reducing method, there is a limitation in controlling particles because the ascorbic acid reduces silver ions even at room temperature, whereas when glucose is used, there is a need for a large amount of polar solvents to adjust the contrast concentration of a silver ion due to a low solubility even in water system, and thus it is difficult to perform a synthesis of high-concentration particles. For these reasons, the conventional silver particle syntheses were only possible at low concentrations (less than 0.05 M), and the amount of uniform particles to obtain from a batch was also limited.
As such, the current methods for synthesizing nanoparticles exhibit a very low production efficiency at the level of diluted concentration conditions, thereby imposing limitations on commercialization, because the nanoparticles tend to aggregate when synthesized at a high concentration. Nanoparticles having a high surface area to volume have a strong aggregative force to exhibit stability, and this force may increase as the concentration of a reactant increases. For this reason, it is difficult to perform a large-scale synthesis of nano-size particles at a high concentration.
As an alternative to solve these problems, Korea Patent No. 10-1423563 discloses a method for producing metal nanoparticles with a high concentration using polyethylenimine (PET) as a reducing agent and a stabilizer. However, the concentration of Ag nanoparticles prepared by the method is 200 mM (i.e., about 20 g/L), which is still insufficient to carry out a large-scale production of metal nanoparticles, and the reaction requires relatively long hours of from 2 to 10 hours, thereby still imposing problems of low production efficiency. Further, the method could not prepare Cu nanoparticles due to weak reducing power of polyethylenimine.
The present inventors have made extensive efforts to resolve such problems of the conventional technologies, and as a result, they could prepare Ag nanoparticles with a high concentration of 500 mM (i.e., about 50 g/L), when polyethylenimine is used together with additional reducing agent, thereby enabling the preparation of metal nanoparticles with high efficiency due to a short reaction time, and subsequently, enabling the preparation of Cu nanoparticles with a concentration higher than 100 mM, which were difficult to prepare by the conventional methods, thus confirming that it is suitable for large-scale production of metal nanoparticles as water can be used as a solvent facilitating large scale processes. Further, it was confirmed that, depending on process conditions, the size of nanoparticles can be easily controlled, and the crystal structures of the nanoparticles can be stably maintained without changes during a long-term storage, thus completing the present invention.